Glass smelted with an auto-smelter and dropped by a dropper can't be picked back up by player or hopper

I had an auto-smelter setup and i put in some glass, the first time i did so i only got 24 glass out of 128 back. After putting 128 more in, i found that the glass had been dropped into the water stream but i couldn't pick it up despite having space. The hoppers also failed to pick it up.